Re: [PATCH v2] md linear: fix a race between linear_add() and linear_congested()

[Shaohua Li <shli@kernel.org>]

On Sat, Jan 28, 2017 at 01:30:09AM +0800, colyli@suse.de wrote:
> Recently I receie a report that on Linux v3.0 based kerenl, hot add disk
> to a md linear device causes kernel crash at linear_congested(). From the
> crash image analysis, I find in linear_congested(), mddev->raid_disks
> contains value N, but conf->disks[] only has N-1 pointers available. Then
> a pointer deference to a NULL pointer crashes the kernel.
> 
> There is a race between linear_add() and linear_congested(), RCU stuffs
> used in these two functions cannot avoid the race. Since Linuv v4.0
> RCU code is replaced by introducing mddev_suspend().  After checking the
> upstream code, it seems linear_congested() is not called in
> generic_make_request() code patch, so mddev_suspend() cannot provent it
> from being called. The possible race still exists.
> 
> Here I explain how the race still exists in current code.  For a machine
> has many CPUs, on one CPU, linear_add() is called to add a hard disk to a
> md linear device; at the same time on other CPU, linear_congested() is
> called to detect whether this md linear device is congested before issuing
> an I/O request onto it.
> 
> Now I use a possible code execution time sequence to demo how the possible
> race happens,
> 
> seq    linear_add()                linear_congested()
>  0                                 conf=mddev->private
>  1   oldconf=mddev->private
>  2   mddev->raid_disks++
>  3                              for (i=0; i<mddev->raid_disks;i++)
>  4                                bdev_get_queue(conf->disks[i].rdev->bdev)
>  5   mddev->private=newconf
> 
> In linear_add() mddev->raid_disks is increased in time seq 2, and on
> another CPU in linear_congested() the for-loop iterates conf->disks[i] by
> the increased mddev->raid_disks in time seq 3,4. But conf with one more
> element (which is a pointer to struct dev_info type) to conf->disks[] is
> not updated yet, accessing its structure member in time seq 4 will cause a
> NULL pointer deference fault.
> 
> The fix includes 2 parts of modification,
>  1) In linear_add(), update mddev->private with new value before
>     increasing mddev->raid_disks, and to make sure on other CPUs their are
>     seen to be updated in same order as linear_add() does (otherwise the
>     race may still happen), a smp_mb() is necessary.
>  2) RCU stuffs are back, to make sure in linear_add() the oldconf won't be
>     destoried when it is still referenced in linear_congested().
> 
> A question is, by this fix, if mddev->private is update to new value in
> linear_add(), but in linear_congested() the for-loop still tests old value
> of mddev->raid_disks, then the iteration will miss the last element of
> conf->disks[]. My answer is don't worry it, it's OK. the reasons are,
>  - When updating mddev->private, the md linear device is suspend, no I/O
>    may happen, it is safe to missing congestion status of the last
>    new-added hard disk.
>  - In the worst case linear_congested() returns 0 and I/O sent to this md
>    linear device, but the new added hard disk is congested, then the I/O
>    request will be blocked for a while if it just happenly hits the new
>    added hard disk. linear_congested() is in code path of wb_congested(),
>    which is quite hot in write back code path. Comparing to add locking
>    code in linear_congested(), the cost of the worst case is acceptable.
> 
> The bug is reported on Linux v3.0 based kernel, it can and should be
> applied to all kernels since Linux v3.0. I see linear_add() is merged into
> mainline since Linux v2.6.18, maybe stable kernel maintainers after this
> version may consider to pick this fix as well.
> 
> Changelog:
>  - v2: add RCU stuffs by suggestion from Shaohua and Neil.
>  - v1: initial effort.

Neil's idea is to store raid_disks in 'struct linear_conf'. In this way, we
never need to worry about the raid_disks and conf aren't consistent. So the
barrier in linear_add is unncessary.
 
> Signed-off-by: Coly Li <colyli@suse.de>
> Cc: Shaohua Li <shli@fb.com>
> Cc: Neil Brown <neilb@suse.com>
> Cc: stable@vger.kernel.org
> ---
>  drivers/md/linear.c | 29 +++++++++++++++++++++++++----
>  1 file changed, 25 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/md/linear.c b/drivers/md/linear.c
> index 5975c99..4f1690c 100644
> --- a/drivers/md/linear.c
> +++ b/drivers/md/linear.c
> @@ -58,13 +58,15 @@ static int linear_congested(struct mddev *mddev, int bits)
>  	struct linear_conf *conf;
>  	int i, ret = 0;
>  
> -	conf = mddev->private;
> +	rcu_read_lock();
> +	conf = rcu_dereference(mddev->private);
>  
>  	for (i = 0; i < mddev->raid_disks && !ret ; i++) {
>  		struct request_queue *q = bdev_get_queue(conf->disks[i].rdev->bdev);
>  		ret |= bdi_congested(&q->backing_dev_info, bits);
>  	}
>  
> +	rcu_read_unlock();
>  	return ret;
>  }
>  
> @@ -173,6 +175,13 @@ static int linear_run (struct mddev *mddev)
>  	return ret;
>  }
>  
> +static void free_conf(struct rcu_head *head)
> +{
> +	struct linear_conf *conf =
> +			container_of(head, struct linear_conf, rcu);
> +	kfree(conf);
> +}
> +
>  static int linear_add(struct mddev *mddev, struct md_rdev *rdev)
>  {
>  	/* Adding a drive to a linear array allows the array to grow.
> @@ -196,15 +205,27 @@ static int linear_add(struct mddev *mddev, struct md_rdev *rdev)
>  	if (!newconf)
>  		return -ENOMEM;
>  
> +	/* In linear_congested(), mddev->raid_disks and mddev->private
> +	 * are accessed without protection by mddev_suspend(). If on
> +	 * another CPU,  in linear_congested() mddev->private is still seen
> +	 * to contains old value but mddev->raid_disks is seen to have the
> +	 * increased value, the last iteration to conf->disks[i].rdev will
> +	 * trigger a NULL pointer deference. To avoid this race, here
> +	 * mddev->private must be updated before increasing
> +	 * mddev->raid_disks, and a smp_mb() is required between them. Then
> +	 * in linear_congested(), we are sure the updated mddev->private is
> +	 * seen when iterating conf->disks[i].
> +	 */
>  	mddev_suspend(mddev);
> -	oldconf = mddev->private;
> +	oldconf = rcu_dereference(mddev->private);
> +	rcu_assign_pointer(mddev->private, newconf);
> +	smp_mb();
>  	mddev->raid_disks++;
> -	mddev->private = newconf;
>  	md_set_array_sectors(mddev, linear_size(mddev, 0, 0));
>  	set_capacity(mddev->gendisk, mddev->array_sectors);
>  	mddev_resume(mddev);
>  	revalidate_disk(mddev->gendisk);
> -	kfree(oldconf);
> +	call_rcu(&oldconf->rcu, free_conf);

we have a handy kfree_rcu just for this.

Thanks,
Shaohua